One of the twin domes of Sir Christopher Wren's masterpiece, the Old Royal Naval College, Greenwich. Originally built as a hospital for sailors, it dates back to the 1700s. (See here for more info). It is used now by the University of Greenwich and Trinity College of Music, with parts open to tourists. In the background you can just see The Queen's House. Originally the home of Charles I's wife Queen Henrietta Maria, it is now a museum and gallery.